An internal combustion engine refers to an engine in which combustion gas produced by combusting fuels directly apply to a piston or a turbine blade and the like to change thermal energy that fuels have to a mechanical work. This engine frequently means a reciprocating engine in which a mixed gas of fuels and air is ignited and exploded inside the cylinder to move a piston, but gas turbine, jet engine, rocket engine and the like are also included as the internal combustion engines.
According to the type of fuels, the internal combustion engine may be classified into gas engine, gasoline engine, petroleum engine, diesel engine, and the like. For instance, the petroleum engine, gas engine, and gasoline engine are ignited by electric spark from a spark plug, and the diesel engine injects a fuel into high-temperature and high-pressure air to ignite the fuel spontaneously. Depending on the stroke and movement of piston, the internal combustion engine may also be 4-stroke or 2-stroke cycle types.
Typically, the internal combustion engine of a vehicle has been known to have thermal efficiency of about 15% to 35%, but even at the maximum thermal efficiency of the internal combustion engine, about 60% or greater of the total thermal energy is consumed due to heat energy, exhaust gas or the like which are discharged to the outside through walls of the internal combustion engine.
Thus, when the amount of thermal energy discharged to the outside through the walls of the internal combustion engine is reduced, the efficiency of the internal combustion engine may be increased. Therefore, methods of installing a heat insulating material to the outside of the internal combustion engine or changing a part of the material or structure of the internal combustion engine or developing a cooling system of the internal combustion engine have been used.
In particular, when the discharge of heat generated in the internal combustion engine to the outside through the walls thereof is minimized, then it is possible to increase the efficiency of the internal combustion engine and the fuel efficiency of a vehicle. However, sufficient studies have not actively been made about a heat insulating material, a heat insulated structure or the like that can be maintained for a long period of time in the inside of the internal combustion engine where high-temperature and high-pressure is applied repeatedly.
Therefore, there exist needs for the development of new heat insulation materials that have superior low thermal conductivity characteristics and heat resistance, and can be maintained for a long period of time when applied to the internal combustion engine.